Colour measurement method and colour measurement device

ABSTRACT

A colour measurement device includes a measurement array (MA) which includes: a plurality of illumination arrays ( 20, 30, 40 ) for exposing a measurement spot (MS) on a measurement object (MO) to illumination light in an actual illumination direction ( 2, 3, 4 ) in each case, and a pick-up array ( 50 ) for detecting the measurement light reflected by the measurement spot (MS) in an actual observation direction ( 5 ) and for converting it into preferably spectral reflection factors; and a controller for the illumination arrays and the pick-up array and for processing the electrical signals produced by the pick-up array. The controller is embodied to process the measured reflection factors on the basis of a correction model, such that distortions in the measurement values as compared to nominal illumination and/or observation directions, caused by angular errors in the illumination arrays and/or the pick-up array, are corrected.

BACKGROUND 1. Technical Field

The present invention relates to colour measurement methods and tocolour measurement devices.

2. Background Art

Colour measurement devices of the type under discussion can be embodied,irrespective of the underlying measurement technology, as autonomousdevices or as peripheral measurement devices for use in connection witha controlling computer which evaluates measurement data. Autonomouscolour measurement devices contain all the operating and display membersnecessary for measurement operations and also their own power supply andare in many cases also equipped with an interface for communicating witha computer, wherein both measurement data and control data can beexchanged with the computer. Colour measurement devices which areconfigured as peripheral measurement devices do not generally have theirown operating and display members and are controlled by thesuperordinate computer like any other peripheral computer device. Forcommunicating with a computer, more modern colour measurement devicesare often for example fitted with a so-called USB (universal serial bus)interface, via which in many cases it is simultaneously also possible tosupply power (from the attached computer).

Metallic paints and paints containing effect pigments are being usedmore and more nowadays, not only in the automobile industry. Such paintsshow a significant angular dependence. Paints containing aluminiumflakes, for example, show a significant brightness flop. Paintscontaining interference effect pigments also show differences in colourwhen the observation or illumination direction is changed, for thispurpose. Multi-angle measurement devices have become established formeasuring such paints. Measuring brilliance is a related topic, in whichthe measurement result is likewise angle-sensitive.

Measurement devices which can detect such properties have to be embodiedto illuminate the measurement object in one or more different, exactlydefined illumination directions (nominal illumination directions) and topick up the light reflected by the measurement object from at least oneexactly defined observation direction (nominal observation direction).The observation direction and the illumination direction can be swappedin accordance with the Helmholtz reciprocity theorem. Colour measurementdevices of this type are for example described in great detail in thedocuments EP 2 703 789 A1 and EP 2 728 342 A1.

In the publication “Device Profiling: Managing Global ColourConsistency” by Wilhelm H. Kettler, DFO Conference on Quality Assuranceand Testing Methods 2008, different causes are presented which can leadto measurement errors when using such colour measurement devices. Theseinclude in particular the so-called systematic errors which are due tocertain deficiencies of the device, such as for example faultycalibration. In a lecture entitled “Colour Management” given by WilhelmH. Kettler for FARBE & LACK Seminars Module 2: Deeper Insights intoColorimetry, 25-27 Jun. 2014, Society for Research into Pigments andPaints (FPL), Stuttgart, particular reference is also made to so-calledangular errors which can result from the geometrical conditions of theillumination and observation beam paths and from the apertures of theillumination and observation beam paths. Angular errors, i.e. deviationsbetween actual illumination and observation directions of themeasurement device and corresponding nominal illumination andobservation directions as predetermined by the measurement geometryselected, have a particularly significant effect, precisely whenperforming measurements on samples containing effect paints.

The present invention deals primarily with avoiding and/or compensatingfor or correcting measurement errors caused by such angular errors.

Documents EP 2 703 789 A1 and EP 2 728 342 A1 describe methods andmeasures on how distortions in the measurement values caused by angularerrors in the measurement device can be corrected. The measuresdescribed in these documents, however, require more apparatus and/ormore complex measurement devices and are also relatively elaborate inpurely procedural terms.

In the publication “Making Sense of Measurement Geometries forMulti-Angle Spectrophotometers” by Eric Kirchner and Werner Cramer inColor Research & Application 37.3 (2012), pages 186-198, a formalism isdescribed regarding how paired combinations of illumination directionsand observation directions are unambiguously assigned a direction inwhich an effect pigment flake has to be orientated in a paint in orderto reflect the illumination specularly in the observation direction.This direction is referred to in the literature as the flake normalangle (cf. FIG. 3). Assigning this direction to a combination of anillumination direction and an observation direction is called“transforming into the flake angle space” in the following. Theorientation of the effect pigment flake in the flake angle space, i.e.the flake normal angle, can be the same for multiple combinations of anillumination direction and an observation direction.

It is the intention of the present invention to improve a colourmeasurement method and a corresponding colour measurement device of therespective generic type to the effect that distortions in themeasurement values caused by angular errors in the different measurementchannels (illumination and observation directions) can be corrected in arelatively simple way and without adding to the complexity of the colourmeasurement device, such that the nominal illumination and observationdirections predetermined by the respective measurement geometry areexactly maintained and distortions in the measurement values are thusavoided. Another aim is to improve the degree of match betweenmeasurement values from different colour measurement devices of the samedesign.

SUMMARY

This object on which the invention is based is solved by the colourmeasurement method in accordance with the invention, as characterised bythe features of independent Claim 1, and by the colour measurementdevice in accordance with the invention, as characterised by thefeatures of independent Claim 11. Advantageous embodiments anddevelopments of the colour measurement method in accordance with theinvention and the colour measurement device in accordance with theinvention are the subject-matter of the dependent claims.

With respect to the colour measurement method, the essence of theinvention is as follows. In a colour measurement method, a measurementspot on a measurement object is exposed to illumination light, by meansof a colour measurement device, in at least one actual illuminationdirection of the colour measurement device, and preferably spectralreflection factors of the measurement light reflected by the measurementspot are measured in at least one actual observation direction of thecolour measurement device. The measured reflection factors are correctedwith respect to distortions in the measurement values caused bydeviations between the actual illumination and observation directions ofthe colour measurement device and nominal illumination and observationdirections predetermined by the measurement geometry of the colourmeasurement device. A continuous correction model is formed from themeasured reflection factors and the actual illumination and observationdirections of the colour measurement device, wherein said modelrepresents the connection between the measured intensity of themeasurement light reflected by the measurement spot and changes in thedifference in the illumination and observation directions. The measuredreflection factors are corrected on the basis of this correction modelby incorporating the actual and nominal illumination and observationdirections of the colour measurement device.

Calculating and applying a correction model, in accordance with theinvention, allows a relatively simple correction of the distortions inthe measurement values caused by angular errors, without changing thedesign of the colour measurement device used. The correction model canbe uniformly used for all measurement geometries.

In accordance with one advantageous embodiment of the colour measurementmethod in accordance with the invention, an actual brightness reflectionalso called luminance reflectance factor is calculated from the measuredreflection factors for each paired combination of illumination andobservation directions, and a nominal brightness reflection also calledluminance reflectance factor is calculated from said actual brightnessreflection factors in each case on the basis of the correction model andthe actual and nominal illumination and observation directions of thecolour measurement device. The measured reflection factors are thencorrected on the basis of the actual brightness reflection factors andthe nominal brightness reflection factors.

In accordance with a particularly advantageous embodiment, the actualand nominal illumination and observation directions of the colourmeasurement device are transformed into flake normal angles, and thecorrection model and the nominal brightness reflection also calledluminance reflectance factors are calculated in the space of the flakenormal angles, wherein the nominal brightness reflection, also calledluminance reflectance factors are expediently calculated from therespective residual between the actual brightness reflection, alsocalled luminance reflectance factors and the correction model.

The measured reflection factors are advantageously calculated on thebasis of the difference between the actual brightness reflection, alsocalled luminance reflectance factors and the nominal brightnessreflection, also called luminance reflectance factors.

In order to calculate the correction model, the measurement spot on themeasurement object is advantageously exposed to illumination light in atleast three, preferably at least five different actual illuminationdirections. It is likewise advantageous if the measurement lightreflected by the measurement spot is detected and measured in at leasttwo different actual observation directions.

In one advantageous modification of the colour measurement method inaccordance with the invention, the actual illumination and observationdirections of a target colour measurement device of the same measurementgeometry are used as the nominal illumination and observation directionsfor correcting the measured reflection factors. The colour measurementdevice used can thus be matched to the target colour measurement devicewith respect to the measurement results.

With respect to the colour measurement device, the essence of theinvention is as follows. A colour measurement device comprises ameasurement array which comprises: at least one illumination array forexposing a measurement spot on a measurement object to illuminationlight in an actual illumination direction in each case, and at least onepick-up array for detecting the measurement light reflected by themeasurement spot in an actual observation direction in each case and forconverting it into preferably spectral reflection factors; and acomputer-based controller for the at least one illumination array andthe at least one pick-up array and for processing the reflection factorsproduced by the at least one pick-up array. The colour measurementdevice also comprises means for correcting the measured reflectionfactors with respect to distortions in the measurement values caused bydeviations between the actual illumination and observation directions ofthe colour measurement device and nominal illumination and observationdirections predetermined by the measurement geometry of the colourmeasurement device. The controller is embodied to form a continuouscorrection model from the measured reflection factors and the actualillumination and observation directions of the colour measurementdevice, wherein said model represents the connection between themeasured intensity of the measurement light reflected by the measurementspot and changes in the difference in the illumination and observationdirections. The controller is also embodied to correct the measuredreflection factors on the basis of this correction model byincorporating the nominal illumination and observation directions of thecolour measurement device.

The controller is expediently embodied to calculate an actual luminancereflectance factor, which represents the intensity, from the measuredreflection factors for each paired combination of illumination andobservation directions, and to calculate a nominal luminance reflectancefactor from the actual luminance reflectance factors in each case on thebasis of the correction model and the nominal illumination andobservation directions of the colour measurement device.

The controller is also expediently embodied to correct the measuredreflection factors on the basis of the actual brightness reflection,also called luminance reflectance factors and the nominal, also calledluminance reflectance factors.

The controller is then advantageously embodied to transform the actualand nominal illumination and observation directions of the colourmeasurement device into flake normal angles and to calculate thecorrection model and the nominal luminance reflectance factors in thespace of the flake normal angles.

The controller is also expediently embodied to calculate the nominalluminance reflectance factors from the respective residual between theactual luminance reflectance factors and the correction model.

The controller is also expediently embodied to calculate the measuredreflection factors on the basis of the difference between the actualbrightness reflection also called luminance reflectance factors and thenominal brightness reflection also called luminance reflectance factors.

The colour measurement device advantageously comprises at least three,preferably at least five illumination arrays for illuminating themeasurement spot in different actual illumination directions. The colourmeasurement device advantageously also comprises at least two pick-uparrays for detecting the measurement light from different actualobservation directions.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention is explicated in more detail on thebasis of the drawings, which show:

FIG. 1 a somewhat simplified representation of the essential design ofan example embodiment of the colour measurement device in accordancewith the invention;

FIG. 2 a block diagram of the computer-based controller of the colourmeasurement device;

FIG. 3 a schematic sketch for explicating the flake normal angle;

FIG. 4 an example of a correction function used in the colourmeasurement method in accordance with the invention;

FIG. 5 a sketch for explicating the angular correction used in thecolour measurement method in accordance with the invention; and

FIG. 6 a block diagram of the most important steps in the colourmeasurement method in accordance with the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

The following rule applies to the description of the figures below:wherever individual reference signs are not entered in a figure,reference is made in this respect to the other figures and thecorresponding parts of the description. The term “measurement array” isunderstood to mean the sum of the components of the colour measurementdevice which serve to illuminate a measurement spot on the surface of ameasurement object and to detect the light reflected by this measurementspot and convert it into corresponding electrical signals. The term“device normal” is to be understood to mean an (imaginary) line which isfixed relative to the device and (ideally) perpendicular to the surfaceof the measurement object when using the colour measurement device inpractice, and which defines the centre point of the measurement spot.The term “actual illumination direction” is to be understood to mean thedirection in which the measurement spot is illuminated. Similarly, theterm “actual observation direction” is to be understood to mean thedirection from which the measurement light reflected by the measurementspot is picked up. The terms “nominal illumination directions” and“nominal observation directions” are to be understood to mean theillumination and/or observation directions for which the colourmeasurement device is configured in accordance with its underlyingmeasurement geometry. The actual illumination and observation directionsmay deviate (slightly) from the nominal illumination and observationdirections, for example due to production tolerances. The term “speculardirection” is to be understood to mean the nominal observation directionreflected on the surface of the (planar) measurement object. Amulti-angle colour measurement device has multiple actual illuminationdirections (and also, as applicable, multiple actual observationdirections). The term “measurement plane” is to be understood to mean aplane which extends through the device normal and all the illuminationdirections and the observation direction(s) and also the speculardirection. All the actual and nominal angles specified refer todirections lying within the measurement plane.

In terms of its general design, the colour measurement device inaccordance with the invention is largely similar in design to thedevices disclosed in the documents EP 2 703 789 A1 and EP 2 728 342 A1mentioned in the background. It comprises a housing which accommodates ameasurement array and an electronic controller. A display array isprovided on the front side of the housing. Operating members are alsoarranged on the upper side of the housing. An interface (preferably aUSB interface) for connecting the housing to an external computer issituated laterally on the housing. The lower side of the housingcomprises a measurement opening through which illumination light canexit the interior of the housing and, conversely, through whichmeasurement light can enter the interior of the housing from without(cf. FIG. 1 in EP 2 728 342 A1).

An exemplary embodiment of the measurement array situated in the housingcan be seen from FIG. 1. The measurement array, which is indicated as awhole by the reference sign MA, comprises an arc body 11 which isattached, spatially fixed, in the housing and in which all the opticaland/or photoelectric components of the measurement array MA arearranged—in the example embodiment shown, in four continuous chambers12, 13, 14 and 15. In the example embodiment shown, these componentsconsist of three illumination arrays 20, 30 and 40 and one pick-up array50 comprising a spectrometer 53 to which the measurement light is fedvia a lens 51 and a light conductor 52. The spectrometer 53 itself issituated outside the chamber 15. Each of the three illumination arrays20, 30 and 40 is assigned a lens 21, 31 and 41, respectively. The threeillumination arrays 20, 30 and 40, which typically each comprise a lightsource comprising at least one or more light-emitting diodes exhibitingdifferent emission spectra, respectively illuminate a measurement spotMS on a measurement object MO with parallel bundles of beams via theassigned lenses 21, 31 and 41. The illumination arrays 20, 30 and 40 areeach aligned in an actual illumination direction 2, 3 and 4,respectively, wherein said actual illumination directions ideally matchthe nominal illumination directions predetermined by the underlyingmeasurement geometry, but in practice exhibit a deviation from thenominal illumination directions (which in most cases is very small butnot negligible) due to production tolerances. The pick-up array isaligned in an actual observation direction 5 which likewise ideallymatches the nominal observation direction predetermined by theunderlying measurement geometry, but in practice exhibits a deviationfrom the nominal observation direction (which in most cases is verysmall but not negligible) due to production tolerances. The measurementarray MA as a whole is arranged such that the illumination directionsand the observation direction are situated in a common measurement planewhich also contains a device normal indicated by the reference sign 0.The measurement plane also contains a specular direction 1, away fromwhich—as the reference direction—the angular positions of theillumination directions 2, 3 and 4 and the observation direction 5 areconventionally measured. The example embodiment shown exhibits ameasurement geometry in which the three nominal illumination directions(not shown) extend at an angle of 15°, 45° and 110°, respectively, tothe specular direction 1, wherein the second nominal illuminationdirection coincides with the device normal 0. The nominal observationdirection (also not shown) extends at an angle of 90° to the speculardirection 1 in the example embodiment shown.

The lenses 21, 31, 41 and 51 can also be completely or partiallyomitted. Similarly, illuminating with parallel light is not obligatory.

The measurement array MA can also comprise fewer or more illuminationarrays and also more than one pick-up array, as is for example the casein the colour measurement device described in the document EP 2 728 342A1.

In the example embodiment shown, the illumination and observation beampaths are embodied to be linear. It is however also possible to deviate,i.e. for example deflect by means of mirrors, one or more of the beampaths, for example for reasons of space. It is merely essential for theoptical axes of the beam path portions which lead directly to and/oraway from the measurement spot to lie in a common measurement plane.

The illumination arrays 20, 30 and 40 are controlled by a computer-basedcontroller 100 (FIG. 2). The controller 100 also controls the pick-uparray 50 and/or its spectrometer 53 and processes the latter'smeasurement signals. The controller 100 can display measurement resultson the display array and receive operating commands from the operatingmembers. Via the interface mentioned, it can also communicate with anexternal computer PC and in particular transmit measurement data andreceive commands and control data. Additional details in this respectare described further below in connection with FIG. 2.

Before the colour measurement device is ready for use, it is firstcalibrated on the basis of dark measurements and measurements on a whitetile (white reference) in a way which is known in its own right, whereinthe measurements on the white tile are taken separately for eachillumination array and pick-up array.

As in the known colour measurement devices of this type, the measuringprocess is in principle performed such that a full spectrum comprising amultitude of interpolation points (wavelength ranges with a width of forexample 10-20 nm each) across the wavelength range of interest (in mostcases, the visible spectrum plus the near UV) is recorded separately foreach illumination channel (illumination arrays 20, 30, 40) by means ofthe pick-up array 50. For this purpose, the spectrometer 53 is activated(put on standby for measuring) by the controller 100 for a certain timeframe, and the light source of the respective illumination array isactivated and/or switched on for a particular period of time within thistime frame. The time frame corresponds to the integration time of thespectrometer.

FIG. 2 is a block diagram showing how the individual components of thecolour measurement device in accordance with the invention co-operate.The computer-based controller 100 which has already been mentionedcomprises, as its most important functional units, a micro-controller110, a hardware control stage 120, a spectrometer control stage 130, aprogram memory and/or firmware memory 140, a data memory 150, a USBinterface 160, operating members 170 and a display array 180, whereinthe micro-controller 110 co-ordinates and controls the whole and is alsoresponsible for communicating with an external computer PC which isattached via the USB interface 160.

The hardware control stage 120 actuates the illumination arrays 20, 30,40, i.e., switches the light sources contained within them on and off,respectively. In addition, the hardware control stage 120 also controlsa drive 71 which is arranged in the colour measurement device and usingwhich a white tile 70, which is likewise provided in the colourmeasurement device, can be introduced into and/or removed again from themeasurement beam path of the colour measurement device.

The spectrometer control stage 130 activates the spectrometer 53 andreads the measurement data produced by it, prepares them and convertsthem into digital measurement signals (spectral reflection factors).

The (non-volatile) program memory 140 contains the firmware and/orsoftware necessary for controlling and for preparing data. If the colourmeasurement device is configured as a peripheral device for asuperordinate computer, the programs for controlling and for preparingdata can alternatively also run completely or partially on the externalcomputer.

The (non-volatile) data memory 150 contains parameters which aresubstantially device-specific, such as for example integration times forthe spectrometer and activation times for the individual illuminationarrays, as well as other data necessary for the colour measurementand/or correction method described further below.

To this extent, the colour measurement device described essentiallycorresponds to the colour measurement devices of this type which aredisclosed in the documents EP 2 703 789 A1 and EP 2 728 342 A1, suchthat the person skilled in the art does not require any furtherexplication in this respect.

The present invention is not concerned with the underlying measurementtechnology as such or with evaluating the measurement results but ratherwith the problem of distortions in the measurement results caused byalignment errors and/or angular errors, in particular of theillumination arrays (as compared to an ideal device exhibiting exactlynominally aligned illumination and observation directions). This andeliminating and/or compensating for such measurement distortions inaccordance with the invention is discussed in the following in moredetail on the basis of FIGS. 3 to 6, wherein it is assumed that thecolour measurement device has already been dark-calibrated andwhite-calibrated.

The flake normal angle φ already mentioned at the beginning plays animportant role in the colour measurement method in accordance with theinvention and/or colour measurement device in accordance with theinvention. The flake normal angle φ respectively links an illuminationdirection (incident angle α_(i)) to an observation direction(observation angle α_(o)), as is illustrated in FIG. 3. An effect paintlayer in which a flake F is situated is indicated by the reference signP, wherein the orientation of the flake F and/or the flake normal issuch that the light entering at the incident angle specularly exits inthe observation direction. The flake normal angle φ can be calculatedfrom the respective illumination direction (incident angle α_(i)) andthe respective observation direction (observation angle α_(o)) on thebasis of simple geometrical considerations and laws of refraction, asfollows:φ=(α_(o)′−α_(i)′)/2 wheresin(α_(i)′)=sin(α_(i))*n ₁ /n ₂sin(α_(o)′)=sin(α_(o))*n ₁ /n ₂

where n₁ is the refraction index of air and n₂ is the refraction indexof the (transparent) substrate material of the effect paint P. In otherwords, each specific combination of an illumination direction (incidentangle α_(i)) and an observation direction (observation angle α_(o)) canbe transformed into a corresponding flake normal angle φ (if therefraction index n₂ is known).

In accordance with an underlying concept of the invention, thistransformation into the flake angle space is then used as the startingpoint for correcting the distortions in the measurement values caused byangular errors, wherein it is presupposed that the actual illuminationdirections of the illumination arrays and the actual observationdirection of the pick-up array (or, if there are multiple pick-uparrays, the actual observation directions) of the colour measurementdevice used are exactly known. Said actual illumination and observationdirections of the specific colour measurement device can for example begauged by the manufacturer, wherein the corresponding data are stored inthe colour measurement device. The actual illumination and observationdirections of the specific colour measurement device can however also begauged and stored by the user, by means of methods which are known intheir own right.

In a first step of the colour measurement method in accordance with theinvention, with the colour measurement device on the measurement objectto be gauged, a set of spectral reflection factors are measured, i.e.one spectrum for each of all the (paired) combinations of actualillumination and observation directions. The values thus obtained arereferred to in the following as spectral raw data R_(i)(λ), where theindex i stands for each specific paired combination of actualillumination and observation directions.

The reflection factor can be evaluated for each wavelength or for asubset of wavelengths. Alternatively, an actual brightness reflectionalso called luminance reflectance factor R(φ_(ai)) which represents theintensity of the measurement light is respectively calculated for eachspecific combination i of actual illumination and observation directionsin a suitable representation of the whole reflection spectrum (forexample, the average values across all the wavelengths).

The colour measurement method in accordance with the invention is basedon the physically substantiated assumption that the light emitted by aparticular illumination array changes its intensity, as measured by apick-up array, continuously as the difference in the illumination andobservation directions changes. Accordingly, a continuous parameterisedmodel of the intensity profile across the illumination and observationdirections is formed, by adaption, from all the actual brightnessreflection also called luminance reflectance factors and thecorresponding actual illumination and observation directions. This modelis referred to in the following as the correction model or correctionfunction. The first co-ordinate of the correction function is given bythe illumination and observation directions, and the second co-ordinaterepresents the intensities measured in said illumination and observationdirections. The correction function unambiguously assigns eachcombination of an illumination direction and an observation direction a(brightness) reflection factor. With the aid of the correction functionand the residual between given points and/or actual brightnessreflection factors and the correction function, each combination of anominal illumination direction and a nominal observation direction isthen unambiguously assigned a nominal brightness reflection factorwithin the definition range of the correction function.

In a preferred embodiment of the invention, the first co-ordinate, i.e.the argument of the correction function, is defined in the flake anglespace by the flake normal angle φ. By transforming the illumination andobservation directions into the flake angle space, it is possible tosimultaneously correct measurements from multiple pick-up arrays. Theaspecular angle can be used as an alternative to the flake normal angle.The aspecular angle is defined in air and calculated for a measurementgeometry from the angular difference between the observation directionand the direction of the specular reflection of the illumination.

The preferred embodiment of the method in accordance with the invention,which operates in the flake angle space, is explicated more specificallyin the following.

For each specific combination i of actual illumination and observationdirections, a corresponding actual flake normal angle φ_(ai) iscalculated. The refraction index n₂ of the measurement object isincorporated into this calculation, as explicated further above, andwould therefore have to be known and/or measured. The refraction indexmeasures about 1.5 for typical effect paints. For performingmeasurements on such effect paints, an accuracy which is sufficient forpractical purposes is satisfied if this value is assumed. Given theseassumptions, the actual flake normal angles φ_(ai) can however also bedetermined before the actual colour measurement and/or even by themanufacturer and stored in the colour measurement device.

Each actual brightness reflection factor R(φ_(ai)) measured (and/orcalculated from the measurements) is assigned to the correspondingcalculated (and/or stored) actual flake normal angle φ_(ai). Thisassignment is shown in a graph in FIG. 4, wherein the crosses which arenot labelled represent the individual actual flake normal angles andtheir assigned brightness reflection factors, i.e. FIG. 4 shows theactual brightness reflection factors in the flake angle space.

It can be seen from FIG. 4 that the reflection factors rise very sharplyat flake normal angles below about 5°, while they initially dropslightly further and then remain practically constant at flake normalangles over about 5°. Correspondingly, angular errors have asubstantially greater effect at very small flake normal angles than atrelatively large flake normal angles.

A correction function C(φ) is then determined from the actual brightnessreflection factors R(φ_(ai)) and the corresponding actual flake normalangles φ_(ai). The correction function is determined and/or adapted inaccordance with any equalisation method which is known in its own right,such that it matches the interpolation points, as given by the actualbrightness reflection factors, as well as possible. The raw data arethen corrected (in multiple steps) on the basis of the correctionfunction C(φ) determined in this way.

For each combination i of the actual illumination and observationdirections, a corresponding nominal flake normal angle φ_(ni) isinitially calculated from the respectively corresponding combination ofnominal illumination and observation directions. The refraction index n₂of the measurement object is again incorporated into this calculationand would therefore have to be measured. As when calculating the actualflake normal angles φ_(ai), however, an accuracy which is sufficient forpractical purposes can also be achieved in this case by assuming a valueof 1.5. Given these presuppositions, the nominal flake normal anglesφ_(ni) could also be determined even by the manufacturer and stored inthe colour measurement device.

A nominal brightness reflection factor R(φ_(ni)) is then calculated fromeach actual brightness reflection factor R(φ_(ai)). This can be achievedby means of the correction function C(φ), for example in accordance withthe relationship:R(φ_(ni))=C(φ_(ni))+[R(φ_(ai))−C(φ_(ai))],

where C(φ_(ni)) is the value of the correction function C(φ) for thenominal flake normal angle φ_(ni), and C(φ_(ai)) is the value of thecorrection function C(φ) for the actual flake normal angle φ_(ai). Thisis illustrated by FIG. 5. The expression [R(φ_(ai))−C(φ_(ai))]represents the residual between the respective actual brightnessreflection factor R(φ_(ai)) and the correction model and/or correctionfunction C(φ).

In a simpler implementation, the local pitch of the flake normal anglesφ_(ni) and φ_(ai) in the correction function C(φ) of FIG. 4 can be takenas the difference C(φ_(ni))−C(φ_(ai)).

The nominal brightness reflection factors R(φ_(ni)) calculated in thisway are then adduced in order to correct the raw data R_(i)(λ), whereineach spectrum R_(i)(λ) recorded for a specific combination i of actualillumination and observation directions is of course correctedseparately on the basis of the nominal brightness reflection factorR(φ_(ni)) calculated for the respective combination.

The spectral raw data R_(i)(λ) can for example be correctedmultiplicatively (through scaling) in accordance with the formula:R _(ic)(λ)=R _(i)(λ)*[R(φ_(ni))−R(φ_(ai))]/R(φ_(ai)),

where R_(ic)(λ) is the angularly corrected spectral data sought.

FIG. 6 is a block diagram summarising once again the most importantsteps in the colour measurement method in accordance with the invention.The individual method steps are indicated by the reference signs 210 to280. As explicated further above, the method step 210 can also beperformed by the manufacturer and/or outside the actual colourmeasurement method performed by the user. The method step 230 can beperformed at any point before the method step 250. The method step 260can likewise be performed at any point before the method step 270.

The method steps 220 to 280, which process and/or calculate measurementvalues and other data, are performed subject to the control of thecomputer-based controller 100 of the colour measurement device inaccordance with the invention. The programs necessary for this purposeare stored in the software and/or firmware memory 140 of the controllerand run by the micro-controller 110, i.e. the colour measurement devicein accordance with the invention differs from known devices of this typesubstantially in that it is embodied and/or programmed to perform thedescribed steps of the method in accordance with the invention.

The colour measurement method in accordance with the invention is basedon a physical underlying concept, namely that of knowing the actualillumination and observation directions of the measurement channels ofthe measurement device and correcting the (spectral) reflectionmeasurement values of the measurement channels on the basis of theinformation on the actual illumination and observation directions andthe nominal illumination and observation directions predetermined by themeasurement geometry and on the basis of a uniform correction model forall the measurement geometries, wherein said correction model is basedon the flake angles. This results in a systematic and plausiblemeasurement value correction.

Using the method in accordance with the invention, the measurement dataare corrected with respect to distortions in the measurement valuescaused by angular errors, such that they practically exactly match themeasurement data recorded by an ideal colour measurement device (i.e.one which does not exhibit any angular errors) of the same measurementgeometry, i.e. the measurement data corrected in this way correspond tothe measurement data which would have been measured if the actualillumination and observation directions matched exactly. The method inaccordance with the invention also however allows a relative correctionwhich adjusts the measurement data to those of another actual targetcolour measurement device (of the same measurement geometry) which issubject to angular errors, wherein the (slightly incorrectly aligned)actual illumination and observation directions of the target colourmeasurement device replace the nominal illumination and observationdirections of the device in accordance with the invention. This relativecorrection is for example advantageous when existing colour measurementdevices are to be supplemented or replaced by new colour measurementdevices, but a significant amount of measurement data have already beenproduced by means of the existing colour measurement devices and thesemeasurement data are also to continue to be able to be used. In thesecases, the new colour measurement devices have to provide measurementresults which are consistent with the existing colour measurementdevices.

The measurement array MA can also be conversely embodied with respect toits illumination and observation arrays. Specifically, this means thatthe measurement object would be illuminated in at least one definedillumination direction only, and the reflected measurement light wouldinstead be detected in three (or more) different observation directionsby means of three or more pick-up arrays. Any combinations of one ormore illumination arrays and one or more pick-up arrays are of coursealso possible.

The colour measurement method in accordance with the invention shows itsstrengths in particular in multi-angle measurements in which themeasurement object is illuminated in different illumination directionsusing a large number of illumination arrays, and the reflectedmeasurement light is measured at different observation directions by twoor more pick-up arrays. The use in accordance with the invention of thecorrection method which is uniform for all the measurement geometriesincreases the number of effective measurement values for the correctionmodel, which enables improved interpolation of the data and increasesaccuracy.

Although the present invention has been described with reference toexemplary embodiments, it is to be understood that the present inventionis not limited by or to such exemplary embodiments.

The invention claimed is:
 1. A method of correcting reflectancemeasurements of a color measurement device having a measurement arraydefining a plurality of measurement channels i, each measurement channeli comprising a paired combination of illumination and observationdirections and each illumination and observation direction having anominal direction based on a measurement geometry of the colormeasurement device, each illumination and observation direction furtherhaving an actual direction as measured from the device as manufactured,comprising: calculating nominal flake normal angles φ_(ni) from thenominal illumination and observation directions of the plurality ofmeasurement channels i; determining actual illumination and observationdirections of a plurality of measurement channels i of the colormeasurement device; calculating actual flake normal angles φ_(ai) fromthe actual illumination and observation directions of the measurementchannels i; measuring actual reflectance spectra R_(i)(λ) for theplurality of measurement channels i at a plurality of wavelengths λ;calculating average actual reflection factors R(φ_(ai)) for theplurality of measurement channels i; determining a correction functionC(φ) on the basis of the actual flake normal angles φ_(ai) and theactual reflection factors R(φ_(ai)); calculating the nominal reflectionfactors R(φ_(ni)) on the basis of the correction function C(φ) and thenominal flake normal angles φ_(ni); correcting the reflectance spectraR_(i)(λ) on the basis of the actual reflection factors R(φ_(ai)) and thenominal reflection factors R(φ_(ni)).
 2. The color measurement methodaccording to claim 1, wherein the method further comprises storing thenominal flake normal angles φ_(ni) and actual flake normal angle φ_(ai)on the color measurement device.
 3. The color measurement methodaccording to claim 1, wherein the correction model C(φ) and the nominalbrightness reflection factors R(φ_(ni)) are calculated in the space ofthe flake normal angles.
 4. The color measurement method according toclaim 3, wherein the nominal brightness reflection factors R(φ_(ni)) arecalculated from the respective residual between the actual brightnessreflection factors R(φ_(ai)) and the correction model C(φ).
 5. The colormeasurement method according to claim 1, wherein the correction modelC(φ) is uniform and is used for all the measurement geometries.
 6. Acolor measurement device with flake normal dependent correction factors,the color measurement device comprising: a plurality of illuminationdevices; at least one pick-up device; the plurality of illuminationdevices and the at least one pick-up device defining a plurality ofmeasurement channels i, each measurement channel comprising a pairedcombination of illumination and observation directions and eachillumination and observation direction having a nominal direction basedon a measurement geometry of the color measurement device, eachillumination and observation direction further having an actualdirection as measured from the device as manufactured; and a controllercomprising a microcontroller and a non-volatile memory; and thenon-volatile memory including: stored nominal flake normal angles φ_(ni)based on nominal illumination and observation directions of each of themeasurement channels i; stored actual flake normal angles φ_(ai) basedon the actual illumination and observation directions of the measurementchannels i; and stored computer executable instructions which, whenexecuted by the microcontroller, cause the color measurement device to:measure actual reflectance spectra R_(i)(λ) for the measurement channelsi at a plurality of wavelengths λ; calculate average actual reflectionfactors R(φ_(ai)) for the plurality of measurement channels i; determinea correction function C(φ) on the basis of the actual flake normalangles φ_(ai) and the actual reflection factors R(φ_(ai)); calculate thenominal reflection factors R(φ_(ni)) on the basis of the correctionfunction C(φ) and the nominal flake normal angles φ_(ni); correct thereflectance spectra R_(i)(λ) on the basis of the actual reflectionfactors R(φ_(ai)) and the nominal reflection factors R(φ_(ni)).
 7. Thecolor measurement device according to claim 6, wherein the instructionsfurther cause the color measurement device to calculate the correctionmodel C(φ) and the nominal brightness reflection factors R((φ_(ni)) inthe space of the flake normal angles.
 8. The color measurement deviceaccording to claim 7, wherein the instructions further cause the colormeasurement device to calculate the nominal brightness reflectionfactors R(φ_(ni)) from the respective residual between the actualbrightness reflection factors R(φ_(ai)) and the correction model C(φ).9. The color measurement device according to claim 6, wherein theinstructions further cause the color measurement device to calculate themeasured reflection factors R_(i)(λ) on the basis of the differencebetween the actual brightness reflection factors R(φ_(ai)) and thenominal brightness reflection factors R(λ_(ni)).
 10. The colormeasurement device according to claim 6, wherein the correction modelC(φ) is uniform and is used for all the measurement geometries.
 11. Thecolor measurement device according to claim 6, wherein the pick-updevice is a spectrometer.
 12. A color measurement device with flakenormal dependent correction factors, the color measurement devicecomprising: a plurality of illumination devices; at least one pick-updevice; the plurality of illumination devices and the at least onepick-up device defining a plurality of measurement channels i, eachmeasurement channel comprising a paired combination of illumination andobservation directions and each illumination and observation directionhaving a nominal direction based on a measurement geometry of the colormeasurement device, each illumination and observation direction furtherhaving an actual direction measured from the device as manufactured; anda controller comprising a microcontroller and a non-volatile memory; andthe non-volatile memory including: stored nominal illumination andobservation directions of each of the measurement channels i; storedactual illumination and observation directions of the measurementchannels i; and stored computer executable instructions which, whenexecuted by the microcontroller, cause the color measurement device to:calculate nominal flake normal angles φ_(ni) from the nominalillumination and observation directions of the plurality of measurementchannels i; calculate actual flake normal angles φ_(ai) from the actualillumination and observation directions of the measurement channels i;measure actual reflectance spectra R_(i)(λ) for the measurement channelsi at a plurality of wavelengths λ; calculate average actual reflectionfactors R(φ_(ai)) for the plurality of measurement channels i; determinea correction function C(φ) on the basis of the actual flake normalangles φ_(ai) and the actual reflection factors R(φ_(ai)); calculate thenominal reflection factors R(φ_(ni)) on the basis of the correctionfunction C(φ) and the nominal flake normal angles φ_(ni); correct thereflectance spectra R_(i)(λ) on the basis of the actual reflectionfactors R(φ_(ai)) and the nominal reflection factors R(φ_(ni)).
 13. Thecolor measurement device according to claim 12, wherein the actualillumination and observation angles are measured by a manufacturer ofthe color measurement device and the corresponding data are stored inthe color measurement device by the manufacturer.
 14. The colormeasurement device according to claim 12, wherein the actualillumination and observation angles are measured by a user of the colormeasurement device and the corresponding data are stored in the colormeasurement device by the user.